1. Field of the Invention
This invention relates to a method for fabricating recorded media such as video discs, digital audio discs and the like.
2. Description of the Prior Art
Several reproducing systems using different types of recorded media are known. One of the known reproducing systems is a so-called electrostatic capacity reproducing system in which there is used an information signal recorded medium where pits are formed in a plane or a groove of the medium according to intended information signals. As the result, the information signals are recorded as changes in geometry. When the recorded information signals are reproduced, a reproducing stylus provided with an electrode is scanned relative to the recorded tracks of the medium. At the time, an electrostatic capacity is established between the electrode of the reproducing stylus and the recorded medium. This capacity varies depending on the geometrical change of the pits, which variation is utilized for the reproduction of the recorded information signals.
As the recorded medium for use in the electrostatic capacity reproducing system of the just-described type, there has been proposed a medium which comprises a thin metallic film, as an electrode, with a thickness of, for example, several hundreds angstrom by which an electrostatic capacity is formed through the electrode of the recording stylus on the surface of the recorded medium which has been press molded as having, for example, an intended change in pits or in a geometrical pattern. The thin metallic film is formed thereon with a dielectric thin film so as to protect the metallic film, to prevent the short-circuiting of the electrodes and to increase the dielectric constant between the electrodes. However, this recorded medium has the drawbacks in that a number of fabrication steps are needed including the step of press-molding a recorded medium itself, the step of depositing a thin metallic film, the step of depositing a thin dielectric film and the like. Accordingly, the fabrication becomes complicate and troublesome with a large-scale fabrication apparatus being needed, leading to a very high production cost.
On the other hand, there is also proposed another type of an information signal recorded medium of the electrostatic capacity type. This medium is made as follows. For instance, starting materials for the medium such as a vinyl chloride resin, a dibutyltin stabilizer, an epoxy stabilizer, silicone oil and conductive carbon black are charged into a mixer and sufficiently mixed therein. The mixture is then kneaded in an extruder for plastics, followed by pelletizing, if necessary. The pellets are press-molded in such a way that information signals are recorded in a geometrical pattern to give an information signal recorded medium of the electrostatic capacity type. The recorded medium of this type is formed, upon reproduction, with an electrostatic capacity between the recorded medium itself and the electrode of a reproducing stylus. Accordingly, the step of depositing a thin metallic film is unnecessary and because fine particles of carbon black are covered with the resin, the step of depositing the thin dielectric film is also unnecessary. Thus, the fabrication of recorded medium is very simple with the production cost being low.
However, when the recorded medium which is made of such a composition including a conductive material such as carbon black is placed under high temperature and high humidity conditions, it deteriorates considerably, leading to the vital disadvantage that defects frequently appear in reproduced information signals at the time of the reproduction of the recorded medium and thus the reproduction operation becomes almost impossible.